Sprinkler heads for an automatic fire extinguishing system normally include a low melting, fusible element having a melting point in the range of 135.degree. F. to 600.degree. F. In the conventional sprinkler head, the fusible element is incorporated as a component of a releasible lever assembly which acts to enclose the water discharge outlet, and when the fusible element melts at the predetermined temperature, the lever assembly is released to thereby open the discharge outlet.
In one common type of sprinkler head, the fusible element is in the form of a link that connects a pair of levers. In a sprinkler head of this kind, the fusible link is under tension stress and has a tendency to cold flow. Cold flow will cause elongation of the link thereby resulting in leakage through the sprinkler head or possible ultimate fracture of the fusible link. With this type of sprinkling head, the loading on the fusible link through use of the compression screw is critical in order to avoid undue tension stress on the loop.
Another common type of sprinkler head is one in which the fusible element is under direct compressive stress. In a sprinkler head of this type the compressive stress can cause cold flow of the fusible metal, and to reduce the tendency for cold flow, the fusible element is normally enclosed in a casing or housing. As the casing has a substantial mass, the time for the fusible element to reach its melting point is considerably increased because of the necessity of heating the mass of the casing. Therefore, the direct compressive stress type sprinkler head, in which the fusible element is contained within a casing of substantial mass, has a relative slow response time.